This invention relates generally to a method and apparatus for controlling molding-process, melt-volume conditions, and more particularly to the control of molding conditions so that molded articles of uniform volumetric consistency and quality are obtained at all times irrespective of fluctuations in melt-flow properties of mold resin in injection molding machines, including injection-molding machines that employ a hot-runner system.
A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever.
The present invention is directed to the control of mold cavity melt conditions in injection molding systems so that molded articles of uniform consistency and quality are produced at all times irrespective of fluctuations in the flow properties of mold resin. The present invention relies upon novel methods and techniques for sensing and monitoring a temperature profile at one or more locations in a molding system. In one embodiment, the invention contemplates the use of an injection molding support sensor array system (machine and mold) throughout a molding process, including start-up, purge, operation, etc.
Heretofore, a number of patents and publications have disclosed systems and methods for the control of injection-molding equipment, the relevant portions of which are hereby incorporated by reference and which may be briefly summarized as follows:
U.S. Pat. No. 5,419,858 to Hata et al., issued May 30, 1995, discloses a system and method for automating the sensing of flow properties of a resin material and the adjustment of molding conditions (e.g., temperature).
The article xe2x80x9cTemperature Control Builds Better Injection Molding, by James R. Koelsch, published in the magazine Quality in May 2000, describes the monitoring and control of temperature as a critical parameter in an injection molding process.
The Dynisco Technical Reference, 42nd Issue, Section Nine xe2x80x9cThe Importance of Accurate Melt Temperature Measurements in Extrusionxe2x80x9d (ref. Pg. 171) states that the thermal degradation of polymers is a time-temperature degradation. The degradation curves are shown therein. The xe2x80x9cVariations In Temperature and Residence Time During Extrusionxe2x80x9d are explained. The importance of accurate melt temperature measurements is in relation to the original material and xe2x80x9cRegrindxe2x80x9d percentage being used. The conclusions are based on a large thermocouple sensor mass that is used at the edge and moved in a melt stream.
In injection-molding machines the cyclic thermal-mechanical operating precision and stability of the equipment has been greatly improved through improvements in the control circuitry used and the use of xe2x80x9creal-timexe2x80x9d closed-loop machine process control. However, the plastic material or xe2x80x9cmeltxe2x80x9d used to mold a part, in the injection molding industry, is produced by a complicated polymerization reaction. The occurrence of some variance in the xe2x80x9cmeltxe2x80x9d and xe2x80x9cflowxe2x80x9d properties of the plastic material cannot be avoided due to variances in the raw material and difficulties in controlling the polymerization reaction. In particular, in resin materials produced by the batch method, maintaining the material properties constant from one batch to another is extremely difficult.
For example, the value of the melt-flow index (MFIxe2x80x94determined using a five minute static state and five minute xe2x80x9cmeltxe2x80x9d extruding time test) often fluctuates by approximately 10% with respect to the specified value for a particular material. Furthermore, in the case of a colored material, there is of course a variance in properties from one color to another due to differences in the pigments and the compounding of additives.
Even if the control precision of an injection-molding machine is improved, a disparity of density, and quality, in the molded articles develops because a fluctuation in resin xe2x80x9cmelt-flowxe2x80x9d effects the xe2x80x9cshrinkxe2x80x9d properties. In particular, a fluctuation in the quality (dimension, weight, density, warping etc.) of the molded articles arises when resin xe2x80x9cmelt-flowxe2x80x9d lots are changed over from one to another. Accordingly, a technician must often monitor the molding machine (e.g., FIG. 2A, 198) and mold temperature at all times and address any fluctuation in resin xe2x80x9cmeltflowxe2x80x9d properties. And the technician must try to adjust for the melt process variance. The molding process is a cyclic sequence starting from an xe2x80x9cOPENxe2x80x9d static xe2x80x9cfreexe2x80x9d thermal state, to a dynamic xe2x80x9cCLOSExe2x80x9d thermal-mechanical injection state, and then followed by a mold xe2x80x9cOPENxe2x80x9d to eject the molded part.
An object of the present invention is to automate the melt to mold exchange by monitoring thermal characteristics using a melt-flow temperature sensor(s). Such sensors may include edge temperature sensor(s) and inner melt-flow temperature sensor(s). As a result of Boyle""s law, the resultant pressure-volume temperature xe2x80x9crisexe2x80x9d may be used to monitor the molding system, and to control the process in an acceptable [Min-Mean-Max] Range. It is further contemplated that the temperature profiles may be recorded and analyzed with trend averaging and LAST-cycle readout, so as to contrast each melt process cycle relative to a predetermined temperature-time sequence control points (process limits). In a preferred embodiment, such a process will be able to identify possible xe2x80x9crejectxe2x80x9d parts and divert such parts for further inspection and/or widen the latitude of the process, if the sample is acceptable.
Another object of the present invention is to determine the input material temperature and moisture status after being loaded into the injection system hopper. A hydroscopic material must be properly conditioned by drying, otherwise the process produces parts with moisture xe2x80x9cstreakingxe2x80x9d and xe2x80x9cbrittlenessxe2x80x9d and a commensurate reduction in the expected finished product performance.
Another object of the present invention is to stabilize the final melt/mold cavity volume and consistency of each cavity molded article""s density by monitoring and controlling fluctuation in resin melt-flow property, through a systematic machine support and melt/mold temperature sensor array system. A system employing aspects of the present invention preferably monitors temperature during each OPEN and CLOSE operation, at one or more locations including: melt source nozzle orifice; mold cavity sprue; runner; gate to vent; and through OPEN mold time to part ejection.
The present invention provides a method of monitoring the indirect process support system and direct machine-to-mold melt temperatures, using inner melt and/or edge temperature sensor(s). In a full system monitoring embodiment, monitoring preferably proceeds from initial machine hopper material conditioning, screw return-melt, and melt-flow injection process, and molding stages of each cavity resin melt-flow. The system may further include processes and controls for independently shutting off gates for each mold cavity (e.g., gating) based upon melt temperature profile for an accepted melt-mold cavity volume.
The inventor has further discovered that temperature change impacts the machine applied mechanical clamp force on the melt/mold cavity volume to establish the molded product final thermal-mechanical xe2x80x9cshrinkxe2x80x9d properties. The machine and mold material mechanical Modulus of Elasticity xe2x80x9cExe2x80x9d (Force per unit area) lowers with increasing temperature, while the material thermal coefficient of expansion xe2x80x9cexe2x80x9d (change in Length divided by initial Length times temperature change) rate increases with increasing temperature. Therefore, the temperature rise increases the material thermal xe2x80x9cstrainxe2x80x9d (Length increase) and lowers the mechanical modulus (strength decrease).
In a typical molding cycle, molten material (melt-flow) exits a nozzle orifice and enters the mold sprue, the runner, and then passes through a small, gate restriction to fill and pack a cavity volume and cure, to form a product of varying density. The nozzle and each cavity gate orifice melt-flow length will vary with a lower viscosity (hotter) inner melt front and a higher viscosity (cooler) edge density characteristic. In accordance with the invention, the melt-flow inner and/or edge temperature sensor arrays measure the melt inner DYNAMIC to STATIC outer edge thermal exchange rate, in real time. Providing a gate melt shut-off means to each cavity that is responsive to a temperature sensor(s), preferably cuts off the machine injection process to maintain consistent cavity melt pack volume.
Engineering thermoplastics and new metal molding materials are processed at high temperatures and require close temperature control. The initial xe2x80x9czeroxe2x80x9d melt injection pressure begins to rise as the melt fills the mold cavity, and the pressure rises to a maximum level (or set-point) during the final pack volume, in the enclosed mold-cavity volume. Furthermore, the resin melt volumetric Bulk Modulus of Elasticity xe2x80x9cKxe2x80x9d varies from the molded part outer surface or xe2x80x9cskinxe2x80x9d to the inner center section, during each melt to mold cavity surface temperature cycle exchange. The volumetric Bulk Modulus xe2x80x9cKxe2x80x9d ratio of the hydraulic oil injection pressure (Ko=1% per 1,000 psi), steel machine/mold clamp force (Ks=⅓% per 1,000 psi) imposed during the machine/melt/mold volumetric exchange phases define the molded product xe2x80x9ccuredxe2x80x9d melt material Bulk Modulus of Elasticity xe2x80x9cKmp.xe2x80x9d
Methods of monitoring a molding process according to the present invention comprise measuring a thermal melt-flow profile, using at least one temperature sensor, where the measurement of temperature may be employed in a nozzle. For example, where the orifice melt start xe2x80x9ctriggerxe2x80x9d temperature set-point. An initialized system trigger, which may be time-dependent causes the system to monitor and store time profiles for a melt temperature profile as seen in FIG. 4, including a rise to peak and fall before an end scan time signal. Monitoring the temperature over a plurality of molding cycles, e.g., for each xe2x80x9cOPENxe2x80x9d purge and xe2x80x9cCLOSExe2x80x9d inject cycle of the molding melt-flow process, allows the system to characterize operation of the molding system and mold.
To prepare a melt shot size, a screw with angular flites and grooves is rotated within a heated barrel to set a barrel melt volume (BmV) shot size. The screw return time depends on the design groove depth, angle and shut off ring melt-flow area. Material is drawn from the hopper, into a barrel aperture, surrounded by a cooling water jacket. The material pellets slide forward within grooves in the rotating screw flites and pass the barrel rear (Br), center (Bc), and front (Bf) heated sections. The screw rotates and translates rearward in the barrel to a fixed screw length position. A hydraulic back-pressure applied to the retracting screw piston inputs added work heat into the contained material. A melt shot size is thereby produced ahead of the screw shut off ring. As the melted material is ejected via force applied to the barrel screw, the barrel melt volume exits a nozzle orifice and is injected into a closed mold cavity volume.
In one embodiment, the temperature sensor may be applied to the nozzle orifice to profile the exiting melt material. In accordance with an aspect of the present invention, the melt temperature-time profile starts when the melt xe2x80x9crisexe2x80x9d temperature trigger set-point (iT1.1) is reached, to initialize a melt scan time (t1.1). When a second (preferably higher) temperature set-point (T1.2) is reached a second time (t1.2) is read. The differential melt rise time (xcex94t1r) is for a fixed melt temperature differential. The changes in melt trigger xe2x80x9crisexe2x80x9d time (T1.2xe2x88x92T1.1=xcex94T1r), indicates the melt viscosity.
As will be described, the present invention includes a method to determine, with a thermocouple sensor array, the fluctuation in resin flow volume for a constant-volume melt-flow process. In accordance with the invention, it is possible to determine whether a xe2x80x9chotterxe2x80x9d melt-flow or a xe2x80x9ccoolerxe2x80x9d fluctuation of the melt-flow occurs, for the same injection molding process parameters. This makes it possible to identify a xe2x80x9cMin-Mean-Maxxe2x80x9d melt temperature-time profile and a method to correlate the melt conversion and molding process to the molded product. Also, aspects of the present invention may be employed to identify the xe2x80x9cLowxe2x80x9d limit and xe2x80x9cHighxe2x80x9d limit in a molding process range, and whether the process latitude can be expanded or process limit maintained. Accordingly, the present invention further includes a method for monitoring and controlling fluctuation in melt-flow in an injection-molding machine, via the measurement of the injection process time for the molding cycle.
The steps preferably include obtaining the degree of fluctuation in the measured melt-flow temperature from the OPEN xe2x80x9cstaticxe2x80x9d to the CLOSE xe2x80x9cdynamicxe2x80x9d mold states. A temperature trigger set-point is initialized to profile the temperature-time for both OPEN mold position melt xe2x80x9cpurgexe2x80x9d and CLOSE xe2x80x9cinjectxe2x80x9d melt/mold flow cavity volume.
It will be further appreciated that the melt-flow front, for example at the nozzle orifice, has an outside static xe2x80x9cEdgexe2x80x9d (exe2x80x2) and an xe2x80x9cInnerxe2x80x9d (i) dynamic melt-flow that stops with melt front xe2x80x9cfreeze offxe2x80x9d (cavity short) or machine injection cut-off. A hydraulic pressure is applied to the piston at the rear end of the machine screw. The hydraulic pressure to screw melt injection pressure is proportional to the ratio of areas, usually 10:1. By applying a 1,000 psi hydraulic pressure to the screw, the internal barrel melt shot pressure (with a closed nozzle orifice) approaches 10,000 psi. The nominal machine hydraulic pressure is 2,000 psi.
The screw xe2x80x9cpurgexe2x80x9d or xe2x80x9cinjectxe2x80x9d melt-flow length establishes the orifice exit melt-flow volume. The barrel melt-flow rate increases in response to a reduction in the nozzle orifice area and each cavity gate area. The melt-flow rate changes the ratio of barrel area squared divided by the nozzle orifice area squared. Each cavity gate melt-flow rate is the ratio of barrel area squared divided by the gate area squared. Each gate melt-flow ratio may change with temperature and mold opening. As the melt cavity pressure builds, a melt/mold cavity opening is similarly increased.
Just as the melt temperature xe2x80x9crisexe2x80x9d may be used to trigger a gate cut off, a subsequent melt temperature xe2x80x9cfallxe2x80x9d set-point may be employed to reinitialize the melt OPEN portion of the cycle. The first melt xe2x80x9cfallxe2x80x9d trigger initializing the time from scan start time. The melt xe2x80x9cfallxe2x80x9d time from the initial melt-flow trigger xe2x80x9cstart,xe2x80x9d and time change differential, indicates the degree of fluctuation for the total melt-mold process. And the temperature is monitored to determine if the melt temperature or time-temperature profile exceeds a predetermined limit (value) of xe2x80x9ctimexe2x80x9d and xe2x80x9ctemperaturexe2x80x9d from the initialized melt trigger set-points. The step of controlling the xe2x80x9cCLOSExe2x80x9d of each cavity by a gate xe2x80x9cshut-offxe2x80x9d action is made such that the actual melt injection xe2x80x9cVOLUMExe2x80x9d will approach a standard injection xe2x80x9cVOLUME.xe2x80x9d If the degree of fluctuation in a melt (e.g. faster xe2x80x9chotterxe2x80x9d or slower xe2x80x9ccoolerxe2x80x9d) is maintained within a predetermined melt/mold cavity flow volume the third pressure-volume rise temperature set-point signals the transition from the impinge fill to pack state.
According to the present invention, there is provided an apparatus for monitoring and controlling a process fluctuation in a mold cavity melt volume by a temperature sensor array property located in the mold cavity halves, the injection molding machine, and nozzle orifice.
One or more thermocouples located in the molding machine nozzle orifice area the nozzle extension and/or the sprue generate the initial output signal representing the melt-flow rise xe2x80x9ctrigger,xe2x80x9d to start the injection process. A mold sprue, runner, gate, vent and fill/pack temperature melt sensor array measures the actual melt/mold cavity melt-flow temperature-time sequence in a molding cycle.
Each mold cavity surface (tab) gate or sub-surface (tunnel) gate shut-off is suitable for stopping the delivery of a resin from the injection-molding machine into each cavity. One or more thermocouples in the melt temperature sensor array may be used as the triggering means. By measuring the resin edge temperature and inner melt front temperature via a melt impinge or inner sensor independent mold control can be achieved. The melt/mold sensor array injection-time measuring means of the present invention is a melt-flow temperature-time trigger sequence. The melt-flow volume is delivered from a nozzle orifice area into a mating mold sprue.
The sprue enters a closed mold cavity volume, created by the mold sprue xe2x80x9cAxe2x80x9d side and eject xe2x80x9cBxe2x80x9d side, created by a machine clamp force. The two mold halves preferably have a melt impinge and edge sensor array. In the mold OPEN position, the exposed cavity is read. In the CLOSED mold position, the created mold cavity volume and melt-flow input molding process inject and cure is read to mold the product. The product mold OPEN to part eject time is also measured.
The initial mold cavity area (cA) can be changed thermally by an increase (cAxc2x7[1+(cAxc2x72exc2x7+xcex94T)]) or decrease (cAxc2x7[1+(cAxe2x80xa22exc2x7xcex94T)]) in mold cavity temperature. The cavity must be physically xe2x80x9ccustomizedxe2x80x9d to match the melt-mold process xe2x80x9cshrinkxe2x80x9d parameters, to achieve the desired final molded product dimension.
The inner melt sensor array measures the temperature-time mold melt-flow length, volumetric heat content and mold cavity heat exchange rate. The edge sensor array measures the melt/mold cavity temperature xe2x80x9crisexe2x80x9d to peak and xe2x80x9cfallxe2x80x9d rate. The melt inner (impinge) to edge temperature xe2x80x9crise-peak-fallxe2x80x9d differential characterizes the molded part process to product thermal strain. The larger the temperature differential, the larger the thermal xe2x80x9cstrainxe2x80x9d in the final molded part.
The present molding process systems use machine hydraulic pressure, barrel and/or mold cavity melt pressure, ram position, and mold opening to stop mold cavity melt-flow. There are hot runner mold valve gate systems to stop the mold cavity melt-flow volume, as described, for example, in U.S. Pat. No. 5,419,858, issued May 30, 1995 for a xe2x80x9cMethod For Controlling Fluctuation In Flow Property Of Resin In Injection Molding Machine.xe2x80x9d
The present invention preferably controls each cavity melt volume using its associated gate melt impinge sensor as a gate trigger for the third pressure-volume xe2x80x9crise.xe2x80x9d The melt impinge sensor array vent triggers the second pressure-volume xe2x80x9crise,xe2x80x9d whereas the melt impinge sensor array fill to pack pressure-volume temperature xe2x80x9crisexe2x80x9d signals the packing of the cavity. As the melt packing pressure acts on the mold parting line, the inner melt sensor moves during the inner melt temperature xe2x80x9crisexe2x80x9d and enhances the inner impinge melt sensor array (iS-A) sensing of the mold part-line opening. The part line opening is a direct measurement of added mold melt volume (mV). An excess of melt may result in the mold cavity overflowing or xe2x80x9cflash.xe2x80x9d
Each of the thermoplastic melt materials has a varying center density, such as the structural foams, polyurethane two-part foam processes, low density thermoplastic elastomer (TPE), high density metal injection molding, and high density thermosetting materials and can be profiled, where the force sensor fails. In accordance with an aspect of the present invention, each mold cavity may be implemented with a gate shut off means (preferably of a shear/punch type) that may be controlled independent of the traditional machine xe2x80x9cmelt-flowxe2x80x9d injection process delivery time. By using a mold gate shut-off for each cavity, the resin xe2x80x9cmelt-flow sensor arrayxe2x80x9d temperature-time profile for each cavity volume is maintained by an independent individual cavity melt-flow injection end time versus the machine injection end time for a total melt-flow cavity fill-pack volume. When the melt-flow temperature sensor has determined that the degree of required fluctuation is achieved (sensing temperature peak caused by cavity fill-pack pressure increase), the cavity melt-flow is stopped, before exceeding a predetermined range.
In accordance with the present invention, the injection pressure actually applied to the resin melt in an injection molding machine 198 is sensed by the impinge melt sensor array (iS-A). In the middle of the melt-flow, the melt temperature measured by the impinge (i) sensor changes as the melt injection pressure changes. This is in accordance with the thermodynamic xe2x80x9cPV/Txe2x80x9d [(Pressurexc3x97Volume)/Temperature] relationship of machine to mold volumetric exchange.
In accordance with the present invention, control is performed in such a manner that the resin melt volume is maintained by a shorter gate open time (to reduce the resin melt volume) and independent melt/mold flow injection time, in comparison to the standard machine injection time. Conversely, control is performed to lengthen the resin melt gate open time, or alarm if the machine system expires. As a result of such gate shut-off control, each mold cavity volume is independent of the machine injection time (i.e., resin flowability) to maintain, in each cavity, a substantially constant melt-flow volume. The standard melt-flow volume mentioned here refers to variable injection times measured under varying molding conditions in which molded articles exhibiting excellent quality are obtained. Accordingly, molded articles of excellent melt-flow volume quality are obtained at all times even if there is a fluctuation in the properties of the resin.
A further object of the present invention includes a method of controlling the resin melt/mold cavity volume fluctuation independent of the injection molding machine. By correlating the machine screw output volume to the position in the barrel during the mold cavity melt fill, any xe2x80x9clossxe2x80x9d in injection melt volume efficiency is indicated. As wear is generated on the screw flites, shut-off ring, and barrel diameter, the melt-flow rate decreases. At a certain point the melt/mold cavity volume falls off to create a xe2x80x9cshortxe2x80x9d molded part. Ref. U.S. Pat. No. 5,419,858, May 30, 1985. The degree of fluctuation determined between the typical machine hydraulic pressure and temperature of the exit melt volume determines the degree of melt-flow rate (volume per unit time) fluctuation. A faster initial hotter melt trigger xe2x80x9cstartxe2x80x9d time and smaller xe2x80x9crisexe2x80x9d time has a faster melt-flow and higher maximum temperature requires shortening inject time and/or lowering the applied hydraulic pressure. Conversely, a slower initial cooler melt trigger xe2x80x9cstartxe2x80x9d time and larger xe2x80x9crisexe2x80x9d time has a slower melt-flow and lower maximum temperature requires lengthening inject time and/or raising the applied hydraulic pressure.
In an embodiment of the present invention, the installed process and melt system sensor array temperature-time data is obtained by sampling and is temporarily stored in a memory in accordance with a sampling xe2x80x9cinitialize temperaturexe2x80x9d to xe2x80x9cscan for the maximum temperature-time and minimum temperature-timexe2x80x9d and xe2x80x9cend temperature-timexe2x80x9d sequence.
The techniques described herein are advantageous because the sensors are inexpensive and easy to install in a drilled hole. The smaller the sensor size, the better the sensor response to temperature within a machine barrel assembly and mold cavity assembly, and thereby monitor and control a melt-flow volume molding process. The techniques of the invention are advantageous because they provide a range of temperature and melt sensing alternatives, each of which is useful in appropriate situations. Some of the techniques can be used to monitor the injection-molding process, whereas other may be used to indicate abnormalities in the process or equipment. As a result of the invention, it will be possible to implement a constant-volume molding melt-flow temperature profile process, where the desired molding pressure (and therefore desired volume) is monitored and gate control output signal as a function of a final inner melt temperature rise.